1. Technical Field of the Invention
This invention relates to electrode core plates that constitute materials used in configuring the positive and negative electrode plates that are used in various types of primary cell, and in the various types of secondary cell of which the main representative types are polymer electrolyte secondary cells, non-aqueous lithium secondary cells, and alkaline secondary cells, together with a method of fabricating such core plates, and cells wherein such core plates are employed.
2. Description of Related Art
Most of the primary cells used in portable equipment are either manganese dry cells, alkaline manganese dry cells, or lithium cells. In the field of secondary cells, nickel-cadmium storage cells and nickel-hydrogen storage cells have been in wide use for some time. In more recent years, lithium ion secondary cells using organic electrolytes, lithium secondary cells using polymer electrolytes, and lithium secondary cells using solid electrolytes, all of which feature lighter weight, have begun to be used.
It is now being demanded in these cells, in recent years, that they perform in a way that exhibits higher energy density, and the appearance of cells exhibiting higher volume-energy density (Wh/l) and weight-energy density (Wh/kg), which are indices of how compact and how light-weighted the cells are, is eagerly awaited. The main factor in determining energy density in battery cells is the active material in the positive and negative electrodes that constitute the power generating elements, but another very important problem is that of how to improve the electrode core plates that retain the active materials in the electrode plates and collect the electrical current. In other words, the energy density of the cells can be improved if the electrode core plates are made thinner and lighter in weight without impairing the current collection characteristics that bear upon electrode reaction utilization factor and the ability to retain the active material.
Typical electrode core plates that have been used conventionally are sintered substrates, porous metal foam substrates, flocculated substrates, perforated corrugated substrates, punched metal substrates, expanded metal substrates, and metal foil substrates. Among these, the first four types, namely sintered substrates, porous metal foam substrates, flocculated substrates, and perforated corrugated substrates, result, after processing, in electrodes of comparatively large thickness, i.e. 0.5 mm or greater. The last three types, namely punched metal substrates, expanded metal substrates, and metal foil substrates, on the other hand, yield, after processing, electrodes of comparatively thin thickness.
The sintered substrate is made by forming nickel or other metal powder into a substrate form and sintering it. The holes in the sintered body are filled with an active material and the substrate is used, for example, as the positive electrode in an alkaline storage cell. The advantage of this sintered substrate is that, as an electrode plate, it exhibits outstanding current collection characteristics and active material retention ability. In the porous metal foam substrate, a metal such as nickel is fashioned into a sponge-like three-dimensionally porous body. This type of substrate is currently used, for example, in the positive electrodes of high-capacity alkaline storage cells. In the flocculated substrate, the surface of a metal sheet is flocked with metal fibers. In the perforated corrugated substrates disclosed, for example, in Japanese Published Unexamined Patent Applications No.7-130370 and in No.7-335208, burrs are formed around punched holes, from one or both sides of a metal plate, which is then subjected to corrugation molding. By adopting these substrates in the coated electrodes on which an active material is coated, it is hoped that improvements will be realized in the current collection characteristics and active material retention ability of the electrodes.
The punched metal substrate is fabricated by subjecting a metal plate to a perforating process using a metal die punch. The expanded metal substrate is obtained by subjecting a metal plate to lath processing. Since both of these substrates are comparatively inexpensive, they are widely used as electrode core materials. The metal foil substrate is used in applications where aluminum, copper, or other metal foil is used as the electrode core plates, and is widely used in thin electrode plates in lithium secondary cells, etc., due to its characteristic of being thin.
What is demanded in these electrode core plates, in addition to low cost and being very amenable to mass production, is that they perform well as electrode plates. More specifically, it is demanded that they exhibit excellent current collection characteristics, excellent active material retention, the absence of sharply pointed projections or burrs that cause separator rupture, resulting in internal shorts between positive and negative electrodes, small volume in the interest of higher energy density, and the ability of the electrolyte or gasses to circulate properly, etc. Among the electrode core plates discussed in the foregoing, however, not a single one satisfies all of these demands in a balanced manner.
More specifically, the sintered substrate exhibits a high base material volume ratio, making it unsuitable for achieving high energy density in electrode plates, and it is also expensive. Both the porous metal foam substrate and the flocculated substrate are comparatively expensive, and each suffers the shortcoming of being susceptible to internal shorting caused by metallic projections. The perforated corrugated substrate retains the active material by projections and burrs, for which reason bonding with the active material is relatively weak, and further suffers the shortcoming of a susceptibility to internal shorting caused by the burrs around the holes. Both the punched metal substrate and the expanded metal substrate have a flat and comparatively simple shape, and therefore are inferior in terms of current collection characteristics and active material retention ability. The metal foil substrate basically has no holes in it, making it problematic in terms of electrolyte liquid circulation and of the current collection characteristics and active material retention exhibited by the electrode plate.
In view of the foregoing, an object of the present invention is to provide electrode core plates that exhibit high active material retention capabilities and good current collection characteristics, with which high energy density can be achieved, and wherewith there is no danger of internal shorting, while also providing a fabrication method therefore and battery cells wherein such electrode core plates are used.
In order to achieve the object stated above, the present invention is a method of fabricating a battery cell electrode core plate that retains an active material and performs a current collection action, comprising: a first process of fabricating, by press molding, a thin metal plate exhibiting electrical conductivity, by means of a plurality of conical or pyramidal punches arranged in a primary molding block and of a plurality of concave dies corresponding to the punches and arranged in a secondary molding block, thereby forming in the metal plate, a plurality of hollow projections that project from one side thereof, and bulges that bulge out in the opposite direction from the projections in the intervals between the projections; and a second process of forming, by subjecting the metal plate whereon are formed the plurality of projections and the bulges to either chemical etching or electrolytic etching, a plurality of through holes by corrosively removing the thin-walled material at the tips of the projections, and forming, at the same time, by corrosion, innumerable minute irregularities over the entire surface of the metal plate.
When this method of fabricating electrode core plates is employed, the simple process of etching is adopted as the second process, wherefore, in the first process, if one merely forms hollow shaped projections wherein the material at the tips is of necessity thinned out, these thin-walled portions will be removed corrosively in the second process and through holes will definitely be formed. When, as in the prior art, a thin metal plate is perforated by a hole punching tool, it is very difficult to punch the holes in a high-density arrangement, the opening ratio rises no higher than 20% or so, and burrs tend to develop. When the method of fabricating electrode core plates according to the present invention is employed, however, the perforations are formed with a high opening ratio of 60%, and can be formed in a high-density arrangement, while, in addition, the corrosion induced by the etching solution does not produce burrs or other sharply pointed places. This method therefore makes it possible to fabricate high-quality electrode core plates, wherewith there is no danger of internal shorting, in a process that is extremely inexpensive and well suited to mass production.
Another method of fabricating a battery cell electrode core plate according to the present invention comprises: a first process of fabricating, by press molding, a thin metal plate exhibiting electrical conductivity, by means of a plurality of conical or pyramidal primary punches and a plurality of concave primary dies arranged alternately in a primary molding block, and of a plurality of concave secondary dies corresponding to the primary punches and conical or pyramidal secondary punches corresponding to the primary dies arranged in a secondary molding block, thereby forming, in the metal plate, a plurality of hollow projections that alternately project from both sides thereof, and bulges that bulge out in the opposite direction from the projections in the intervals between the projections: and a second process of forming, by subjecting the metal plate whereon are formed the plurality of projections and bulges to either chemical etching or electrolytic etching, a plurality of through holes by corrosively removing the thin-walled material at the tips of the projections, and forming, at the same time, by corrosion, innumerable minute irregularities in the entire surface of the metal plate. Thereby, in addition to realizing the same benefits as with the fabrication method described earlier, a battery cell electrode core plate can be fabricated wherein the projections project alternately from both sides.
In the first process in the present invention, as described above, it is possible to form the projections and the bulges with a reciprocal motion, using a setup wherein the primary molding block and the secondary molding block are positioned in parallel. Alternatively, a metal plate that is being moved at a constant speed can be fed in between the primary and secondary molding blocks that are linked together in a roller press configuration, so that a plurality of hollow projections together with a plurality of bulges that bulge out in the opposite direction from the projections are continuously formed in the metal plate. Thus it is possible to mass produce electrode core plate of high quality.
In the present invention as described above, moreover, after the second process has been finished, the metal plate wherein the plurality of projections and bulges are formed can be subjected to press processing from both sides so that some of the bulges are made flat. By so doing, the volume of the electrode core plate can be reduced and high energy density is achieved.
In the battery cell electrode core plate manufactured by the fabrication method of the present invention, projections that project on one side are formed, through holes opened in the thickness dimension of the metal plate are formed inside the projections, bulges that bulge out in the opposite direction as the projections are formed between the projections, and many minute irregularities distributed over the entire surface of the metal plate are formed.
In this electrode core plate, contact with the active material is improved by the presence of the bulges, as compared to where the plate is flat. In addition, due to the numerous minute irregularities, the degree of bonding between the active material and the electrode core plate is improved and, at the same time, the electrolyte liquid retention is improved. Furthermore, the active material particles on both sides of the electrode core plate are directly interconnected via the through holes so that the active material bonding strength is improved while, at the same time, the circulation of the electrolyte and gas is facilitated by the through holes. Accordingly, in electrode plates wherein this electrode core plate is used, both the current collection characteristics and active material retention are excellent. In addition, because the plate is formed by an etching process, burrs and other sharp places are the first places to be dissolved, so that they disappear, for which reason the occurrence of internal shorts is suppressed.
In battery cell electrode core plate manufactured by another fabrication method according to the present invention, in the electrically conductive metal plate, projections are provided that project alternately on both sides, through holes are opened in the thickness dimension of the metal plate inside the projections, bulges that bulge out in the opposite direction from the projections are provided in the trunks of the projections, and numerous minute irregularities are provided, distributed over the entire surface of the metal plate. In addition to the benefits of the electrode core plate noted earlier, this electrode core plate is advantageous in that the active material retention capability is further improved because the projections are present on both sides of the metal plate.
In the electrode core plate described in the foregoing, it is preferable that the plurality of through holes are configured in a regular distribution, arranged in a two-dimensional lattice form. When that is done, active material retention is uniform all over the metal plate and high current collection characteristics are obtained.
It is also desirable that the hole shapes in the through holes be either circular or polygonal.
It is further desirable that the thickness of the battery cell electrode core plate (that is, the apparent thickness that includes the projections either on one side or on both sides of the metal plate) be set at a thickness that is no more than three times the original thickness of the metal plate. When that thickness exceeds three times the original thickness, the condition of contact with the active material is favorable, but the tensile strength is inadequate when processing the electrode plate, resulting in electrode plate breaks and this also readily becomes a cause of internal shorting. If the thickness of the battery cell electrode core plate is expressed as X xcexcm and the opening ratio as Y%, then the configuration should be such that 20xe2x89xa6Xxe2x89xa650 and Yxe2x89xa6X+10.
The electrically conductive material making up the metal plate should preferably be any of iron, copper, nickel, aluminum, or some alloy made up primarily of these metals. It is also desirable that the amounts of the impurity elements silicon and carbon contained in the conductive materials making up the metal plate be limited to 0.2 wt % or less, respectively, in order to prevent a deterioration in the active material utilization factor.
It is moreover desirable that at least some part of the electrically conductive materials making up the metal plate be integrated with another substance that is either a metallic material, an inorganic substance, or an organic substance. This will prevent corrosion being caused by the electrolyte and enhance durability.
The battery cell of the present invention is configured by forming an electrode plate by packing in the active material from both sides of the battery cell electrode core plate of the present invention, and making this electrode plate at least either the positive or the negative electrode. This battery cell will exhibit high current collection characteristics, not be susceptible to internal shorting, and facilitate the attainment of high energy density.